The present invention relates generally to automotive exhaust systems. More particularly, the invention relates to a new method and system for purging a hydrocarbon trap positioned in an automotive exhaust system by supplying air to the exhaust stream.
Certain automotive vehicles are equipped with emission control devices, commonly referred to as hydrocarbon (HC) traps, that adsorb hydrocarbons when the temperature of the device is below a certain level and release and oxidize the stored hydrocarbons when the temperature of the device rises above a certain level. HC traps are particularly useful in a vehicle""s exhaust system in combination with a three-way catalytic converter (a xe2x80x9cTWCxe2x80x9d) positioned upstream of the HC trap. In steady-state operation, conventional three-way catalysts store oxidants (NOx and oxygen) when the engine is operated with a lean air/fuel ratio and release the oxidants when the engine is operated with a rich air/fuel ratio. The released oxidants react with the incoming HC and CO (produced when the engine is operated with a rich air/fuel ratio) to form H2O and CO2. In this way, HC and CO are oxidized and NOx is reduced. However, conventional three-way catalysts are relatively ineffective below a certain temperature. Therefore, HC traps are sometimes used in the same exhaust system with three-way catalysts to store the HC produced by the engine during and after initial start-up and prior to the three-way catalyst reaching a temperature at which it can effectively reduce NOx and oxidize HC and CO.
When the temperature of an HC trap reaches a certain purge threshold temperature, the HC trap begins to release the HC that it stored when the temperature of the HC trap was relatively low. As with three-way catalysts, the released HC reacts with oxygen in the exhaust stream to form H2O and CO2. To minimize the amount of unreacted HC that is emitted into the atmosphere, it is important to ensure that there is sufficient oxygen present in the exhaust stream entering the HC trap to oxidize as much of the released HC as possible.
To ensure that sufficient oxygen is present in the exhaust stream, it is known to use an air pump to supply additional oxygen upstream of an HC trap in the exhaust stream. However, typical air pumps used in automotive applications provide a constant air mass when activated. While the additional air provided by the air pump may be sufficient to oxidize the HC released from the HC trap, the unheated air also tends to lower the temperature of the HC trap. If the HC trap temperature falls too much, it will stop oxidizing the released HC and permit unreacted HC to be expelled into the atmosphere.
The inventors have recognized that a new method and system for purging HC traps is needed that both ensures that sufficient oxygen is supplied to the HC trap and maintains the temperature of the HC trap at a desirable level above the purge threshold temperature.
The present invention is directed to an improved method and system for purging an HC trap by supplying additional oxygen to the HC trap. An internal combustion engine is coupled to an exhaust system that includes a three-way catalytic converter positioned downstream of the engine. A hydrocarbon (HC) trap is positioned in the exhaust stream downstream of the three-way catalyst. An air pump is connected to the exhaust stream between the three-way catalyst and the HC trap and is capable of selectively providing air to the exhaust stream in response to a control signal from an electronic controller. When it is determined that the HC trap has reached its purge temperature threshold, the controller causes the air pump to provide air to the exhaust stream entering the HC trap.
To ensure that sufficient oxygen is supplied to the HC trap without lowering the HC trap temperature to an undesirable level, the air pump is xe2x80x9cpulsedxe2x80x9d so as to provide air to the exhaust stream according to an xe2x80x9con-offxe2x80x9d duty cycle. Specifically, the air pump is turned on for a certain period of time and then turned off for a period of time. The duration of the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d periods are determined based upon the mass airflow in the engine""s intake manifold, which is indicative of the engine load. The xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d durations are selected such that the xe2x80x9conxe2x80x9d periods are long enough to provide sufficient oxygen to the HC trap and the xe2x80x9coffxe2x80x9d periods are long enough to limit the cooling effect of the added air. The xe2x80x9con-offxe2x80x9d duty cycle is repeated until it is determined that the HC trap has been fully purged.